A capacitor is any device having the principal electric property of capacitance, i.e. the ability to store an electric charge. In the field of electronics, the ability of a capacitor to store an electric charge is useful in controlling the flow of an electric current. Moreover, capacitors may be employed in circuits for the purpose of filtering electrical signals; for example, a capacitor with variable capacitance can be used in the tuning circuit of a radio or television receiver. Varying the capacitance changes the resonant frequency of the tuner circuit so that it matches the frequency of the desired station or channel, filtering out signals of all unwanted frequencies.
The simplest of capacitors will comprise two plates of a conductive material separated from one another by an insulator, also referred to as a dielectric, with each plate connected to a terminal. When voltage is placed across the terminals of an uncharged capacitor, charge flows to each of the plates (positive charge to the anode plate and negative charge to the cathode plate) but not across the insulator sandwiched between. As the opposite charges increase on the conductive anode and cathode plates, the force on the dielectric between them also increases, thereby causing the electric field across the dielectric to increase. This phenomenon gives rise to a voltage which increases proportionally with the charge on the plates.
The ratio of the charge magnitude on each plate to the electric potential (voltage) between the plates is the aforementioned capacitance and approximates the externally applied voltage source used to charge the capacitor. When these two voltages have the same magnitude (the voltage source and the capacitor), the current ceases to flow and the capacitor is considered to be charged. A charged capacitor is subsequently discharged by reducing the external voltage through an applied electrical load, thus causing a decrease in the voltage across the plates when a produced current quickly flows the charge off the plates.
There are many types of capacitors, each varying in construction and material combinations, but the physics explained above are essentially the same for all. A common capacitor type employs ceramic for the dielectric layer and may take either a cylindrical structure, wherein a hollow cylinder of the ceramic material is lined with thin films of conductive metal on its inner and outer surfaces, or a flat, parallel plate structure wherein a plurality of plates of ceramic and conductive materials are interleaved to create the sandwiched “electrode-dielectric-electrode” arrangement.
Manufacturing is fairly straightforward for capacitors comprised of the so-called parallel plate structure. A layer of dielectric is sandwiched between two conductive electrode layers, wherein capacitance of the resulting parallel plate capacitor is a function of the overlapped area of the electrode plates, thickness of the dielectric layer, and the permittivity of the dielectric.
A multi-layer ceramic capacitor (MLCC) is a parallel plate capacitor having a plurality of stacked “electrode-dielectric-electrode” arrangements (EDE) that each may form a tri-layer. The capacitance of a MLCC may be drastically increased by the parallel connection of the many parallel plates. Quite simply, more stacked arrangements increases capacitance and forms a MLCC. Similarly, individual capacitors can also be connected in series, essentially spreading the above described MLCC over a larger surface area as opposed to a higher amount of head room.
An advantage of serially connected capacitors over a highly stacked MLCC is that the serial arrangement is known in the art to exhibit better resistance to voltage breakdown (as the charge and voltage on a given capacitor are increased, at some point the dielectric will no longer be able to insulate the charges from each other, subsequently exhibiting dielectric breakdown, or high conductivity in some areas, which tends to lower the stored energy and charge, generating internal heat).
Turning back to the manufacturing methods employed to make typical MLCCs, a capacitor may be made by applying a dielectric slurry, such as a ceramic based slurry, between alternating pairs of conductive plates. However, the manufacturing of MLCCs has largely migrated to the use of a conductive ink or paste (an ink or paste comprising a conductive material such as, for example, silver), in lieu of plates; This ink or paste may be screen-printed over a “green tape” of a dielectric slurry which was previously cast on a carrier polymer film. Consistent with what has been described above, many layers of interleaved dielectric tapes and electrode applications can be stacked and laminated together to form a final MLCC product.
Multi-layer ceramic capacitors with about 500 to about 1000 layers, where the dielectric layers often being less than about 1 micron thickness, are achievable. Reduction in layer thickness in a MLCC directly correlates with saved head room, however, it is often not the headroom that comes at a premium. In actuality, the overall surface area required to accommodate a passive electrical component, such as a MLCC, represents valuable real estate in an electrical circuit.
To reduce the space passive components occupy using surface mount technology, 0402 size (about 0.04 inch by about 0.02 inch) is gaining momentum as the most popular and even 0201 (about 0.02 inch by about 0.01 inch) can be reliably produced. Generally speaking, when holding capacitance constant, the smaller the MLCC is, the better. However, there is a limit to simply reducing the area footprint and increasing layer quantities as continued reduction in the thickness of dielectric and electrode layers can create manufacturing problems. Therefore, there is a need to provide alternate methods to continue the trend to reduce the size and increase the capacitance density of the ceramic capacitor.